Doppler shift source velocity components

1. The problem statement, all variables and given/known data
Why does a source moving perpendicular to the line between it and the observer produce no doppler effect?

2. Relevant equations

3. The attempt at a solution
I have heard the the radial velocity of the source relative to the observer must be zero but i dont understand why, if the distance between the source and the observer is increasing or decreasing , shouldn't there be a doppler effect since the wave length is changing?

i guess my real question is, how does the doppler effect work when the observer and source are not moving in a straight line, what do we take in as the velocity of the observer and the source in the doppler effect equation?